A catheter is a long flexible tube introduced into a blood vessel or a hollow organ for the purpose of introducing or removing fluids; implanting medical devices; or for performing diagnostic tests or therapeutic interventions. Catheters are conventionally known and frequently used; and a wide range and variety of catheters are available which are extremely diverse in shape, design and specific features.
Typically a catheter is a long thin tube of fixed axial length, with two discrete, unique ends. One end is designed and engineered to be inserted in the body; the other end generally remains outside the body, and is so designed. Most catheters have at least one internal lumen of a volume sufficient to allow for on-demand passage of a diverse range of wires, rods, liquids, gases, transmitting energy, fiber optics, and specifically designed medical instruments.
The fundamental principles and requirements for constructing a guiding flexible catheter exist as conventional knowledge in the relevant technical field; and all of the essential information is publicly known, widely disseminated, and published in a variety of authoritative texts. The medical and technical literature thus provides an in-depth knowledge and understanding of the diagnostic and therapeutic uses of conventional catheters and commonly used catheterization techniques. Merely representative of the diversity of publications now publicly available are the following, each of which is expressly incorporated by reference here: Diagnostic And Therapeutic Cardiac Catheterization, second edition (Pepine, Hill, and Lambert, editors), Williams & Wilkins, 1994 and the references cited therein; A Practical Guide To Cardiac Pacing, fourth edition (Moses et. al., editors), Little, Brown, and Company, 1995 and the references cited therein; Abrams Angiography, third edition (H. L. Abrams, editor), Little, Brown & Co., 1983; Dialysis Therapy, second edition (Nissenson & Fine, editors), Hanley & Belfus Inc., 1992; and Handbook of Dialysis, second edition (Daugirdas & Ing, editors), Little, Brown and Co., 1994.
Thus, in accordance with established principles of conventional catheter construction, the axial length of the catheter may be composed of one single layer or of several layers in combination. In most multilayered constructions, one hollow tube is stretched over another tube to form a bond; and the components of the individual layers determine the overall characteristics for the catheter as a unitary construction. Many multilayered catheters comprise an inner tube of Teflon, over which is another layer of nylon, woven Dacron, or stainless steel braiding. A tube of polyethylene or polyurethane typically is then heated and extruded over the two inner layers to form a firm bond as the third external layer. Other catheter constructions may consist of a polyurethane inner core, covered by a layer of stainless steel braiding, and a third external jacket layer formed of polyurethane.
In addition, a number of dual-lumen catheters are known today which differ primarily in the size and spatial relationship between their individual lumens. Typically, a dual-lumen catheter can take many different forms such as: two co-axially positioned lumens where one small diameter tube extends axially through the internal volume of a larger diameter tube; or the catheter is a single large diameter tube and has a centrally disposed inner septum which divides the interior volume into two equal or unequal internal lumens; or where the material substance of the catheter tube contains two discrete bore holes of differing diameters which serve as two internal lumens of unequal volume lying in parallel over the axial length of the catheter. All of these variations present different dual-lumen constructions for catheters having a similar or identical overall diameter size.
Catheters are generally sized by external and internal diameter and length. The internal diameter is specified either by actual diameter (in thousandths of an inch or millimeters or French size). Many newer thin-walled catheter designs provide a much larger internal lumen volume to external diameter ratio than has been previously achieved; this has resulted in catheters which can accommodate much more volume and allow the passage of much larger sized articles through the internal lumen. External diameter is typically expressed in French sizes which are obtained by multiplying the actual diameter of the catheter in millimeters by a factor of 3.1415 (.pi.). Conversely, by traditional habit, the actual size of any catheter in millimeters may be calculated by dividing its French size by a factor of .pi.. As an illustration of size usage. French sizes from 4-8 are currently used for diagnostic angiography. In addition, because of the variation between standard, thin-walled, and super high-flow catheter construction designs, a wide variety of external and internal lumen diameter sizes exist today.
In order to perform effectively in specialized medical procedures and in particular anatomical areas, specific categories or classes of catheters have been developed. Among the presently known specific types of catheters are: peritoneal catheters employed for peritoneal dialysis and which provide dialysate inflow and outflow for the removal of the by-products of metabolism from the blood; acute and chronic urinary catheters introduced into the bladder, the urethra, or directly into the renal pelvis for the removal of urine; central venous catheters are designed for insertion into the internal jugular or subclavian vein; right heart catheters such as the Cournand and Swans-Ganz catheters designed specifically for right heart catheterization; transeptal catheters developed specifically for crossing from right to left atrium through the interatrial septum at the fossa ovalis; angiographic catheters which are used for right or left ventriculography and angiography in any of the major vessels; coronary angiographic catheters which include the different series of grouping including Sones, Judkins, Amplatz, multipurpose, and bypass graft catheters; as well as many others developed for specific purposes and medical conditions.
An illustrative and representative example of traditional catheter usage is provided by the medical specialty of hemodialysis--the process by which extra water and toxic metabolites are removed from the blood by a dialysis machine when the kidneys are impaired by illness or injury. A summary review therefore of renal insufficiency or failure, the technique of hemodialysis, and the role of specialized catheters in machine dialysis will demonstrate and evidence conventional limitations.
A wide variety of pathological processes can affect the kidneys. Some result in rapid but transient cessation of renal function. In patients so affected, temporary artificial filtration of the blood is sometimes necessary. With time, renal function gradually improves and may approach normal; and dialysis is therefore usually required only for a short duration. The time required for the kidneys to recover will depend on the nature and severity of the injury which typically varies from a few days to several months Thus, if the acute condition lasts for more than three or four days, the patient will probably require hemodialysis at least once while awaiting return of renal function.
Other pathological conditions result in a gradual deterioration of renal function over months or years. These patients can go for quite some time before toxic concentrations of metabolites accumulate. Once they reach the stage where dialysis is necessary, however, it is usually required for the rest of their lives. Some of these patients retain low levels of renal filtration and can therefore be dialyzed as infrequently as once a week. Many progress to total renal failure and require hemodialysis two or three times each week. Still other types of renal injury result in rapid onset of permanent renal failure necessitating life long dialysis.
The dialysis machine serves as an artificial kidney to reduce harmful concentrations of the by-products of metabolism and to remove excess water from the blood. The machine is essentially a special filter in series with a blood pump. The filter is connected to the patient via two blood lines. Blood drains from the patient to the dialysis machine through the afferent line; and a volume displacement pump provides suction to assist drainage. The same pump pressurizes the blood to overcome the resistance imposed by the filter. The filter makes use of a semipermeable membrane which separates the blood path from that of dialysate, a special buffered solution used to clear filtered substances. Unwanted molecules diffuse through the semipermiable membrane into the rapidly flowing dialysate and are carried out of the filter in a manner analogous to that of urine flowing through a renal tubule. The membrane is incorporated as multiple pleated sheets or small caliber tubes to increase the surface area across which diffusion may take place. Blood leaving the filter returns to the patient through the second, or efferent, blood line.
The ability to perform dialysis effectively is dependent on high flow of blood through the filter. Furthermore, blood must be returned to the patient as rapidly as it is withdrawn to prevent the hemodynamic consequences of large fluctuations in intravascular volume. It is therefore necessary that both afferent and efferent blood conduits be connected to the patient by way of transcutaneous catheters inserted into large bore, high flow blood vessels.
For patients in whom renal recovery is anticipated, percutaneous intravenous access is used frequently. This technique makes use of a large bore flexible two-lumen catheter. This catheter, measuring 10 French, (roughly 3 mm in diameter) is introduced into the central venous circulation via the subclavian or internal jugular vein. Placement of transcutaneous venipuncture in conjunction with the Seldinger technique and serial dilation is used; and the tip of the catheter is positioned at the junction of the superior vena cava and the right atrium. Alternatively, the catheter is placed percutaneously in the femoral vein. Blood is withdrawn from one lumen and returned through the other. The afferent lumen ends 2 or 3 centimeters from the catheter tip which inhibits recirculation of efferent blood. The large size and high blood flow of the superior vena cava permits very effective dialysis with this technique.
Unfortunately, however, this method of percutaneous intravenous access is not well suited for patients who will require long term or permanent dialysis. The presence of a foreign body (the access catheter) breaching the skin is associated with a high risk of infection. This risk increases with time, and in long term applications, is prohibitive. Because the foreign body is in an intravascular location, the infection is usually associated with sepsis, or infection of the blood stream, which can be lethal. Special catheters are designed to be implanted or "tunneled" subcutaneously for several centimeters to decrease the incidence of sepsis; and if absolutely sterile technique is used when manipulating the catheter (and the skin exit site is meticulously cleaned and dressed), these tunneled catheters can be used for several months without incident. Despite pristine care, however, infection is inevitable with extended use; and all such catheters eventually must be removed. Aside from sepsis, long term central venous access is also associated with a time related increase in the risk of endocarditis, cardiac perforation from catheter tip erosion, and superior vena cava thrombosis. Patients who require permanent or lifetime hemodialysis therefore must be attached to the dialysis machine in a different way.
Two methods have evolved to provide long term vascular access in patients on permanent dialysis. The first method involves surgically implanting a 6 or 8 mm dacron or gortex tube graft subcutaneously in the upper extremity. A small transverse incision is made in the proximal forearm, just below the crease. One end of the tube graft is anastomosed to the side of the brachial artery and the other, to the side of a large antecubital vein. The body of the graft between the two anastomoses is tunneled just below the skin in a horseshoe configuration, with the bend at the mid forearm. Blood flowing through the tube bypasses the capillary bed, and as such, represents a very low resistance pathway. This surgically created "short circuit" in the circulatory system is referred to as a shunt. The low resistance in the shunt results in a high blood flow. To perform hemodialysis, two large bore needles are sterilely introduced into the graft lumen through the intact skin. This can be readily accomplished as the graft, in its subcutaneous location, is easily palpated. The large lumen and high blood flow provide excellent drainage for dialysis. After hemodialysis is completed, the needles are removed, so no permanent breech in the skin exists. Each time the patient is dialyzed, needles are reintroduced.
The second method involves the creation of a direct arteriovenous fistula between the radial artery and an adjacent vein without the use of a prosthetic graft material. Once again, the capillary network is bypassed, and a low resistance "short circuit" in the circulatory system results. The direct and increased volume of blood flow through the fistula leads to massive venous dilation. Dialysis catheters are then introduced into the dilated veins.
To create an arteriovenous fistula for permanent hemodialysis, an incision is made at the wrist and the radial artery identified and mobilized. An adjacent vein is mobilized as well. After obtaining vascular isolation with vessel loops or soft clamps, the artery and vein are opened longitudinally for a distance of 5 to 8 mm. Using fine monofilament suture and magnified visualization, the arteriotomy and venotomy are sewn together, creating a side-to-side anastomosis (or, alternatively, the end of the vein is sewn to the side of the artery). This surgically created connection allows blood to bypass the capillary bed, and results in dramatically increased flow through the forearm veins. In contrast to the shunt technique of the first method, there is no easily palpable prosthetic graft just beneath the skin that can be entered transcutaneously. However, because the arteriovenous fistula is performed at the wrist, the thin-walled forearm veins are subjected to high blood flow; and, over a short period of time, dilate to 2-3 times their initial size. The massively dilated veins are easily identified and can be accessed by two large bore needles as described above for the shunt.
Each of the two surgical techniques has relative advantages and disadvantages. The shunt, although simple to construct, involves implantation of a foreign body. Each time a needle is introduced percutaneously, there is risk of infection of the graft with skin organisms. The risk of infection is not as great as was described for the indwelling intravenous dialysis catheters, but is still present. With meticulous attention to sterile technique, shunts of this type can be maintained for years. Hemodialysis patients often have impaired immune systems, however, and infection requiring shunt removal is not uncommon. A second problem seen with prosthetic shunts is that of thrombosis necessitating thrombectomy or revision. Reactions take place between the prosthetic material and the platelets in the blood that result in liberation of clotting factors. These factors stimulate abnormal growth of the intima, or lining of the vein, at the venous anastomosis. This abnormal growth narrows the anastomosis resulting in decreased flow through the graft and thrombosis. Hemodialysis patients often require multiple operations for thrombectomy and shunt revision throughout their lives to maintain vascular access.
The direct arteriovenous fistula method is highly desirable and advantageous in that no prosthetic material is implanted; and the risk of infection is therefore dramatically reduced. In addition, all blood carrying surface are lined with living intima, and intimal proliferation is very uncommon. Moreover, the vein, being composed of living tissue, has the ability to mend itself and is less likely to form pseudoaneurysms as is occasionally seen with prosthetic shunts after extended use. For these reasons, most surgeons prefer to perform this procedure when it is technically feasible.
Unfortunately, in many patients, use of an arteriovenous fistula is technically not possible by conventional means. As described above, the radial artery is dissected out at the wrist; and a distal dissection zone is preferred in that more veins will be subjected to increased flow and dilation, resulting in more potential sites for hemodialysis needle insertion. However, the radial artery is somewhat small at this distal location which makes anastomosis technically more demanding, especially in smaller patients. Furthermore, because a direct anastomosis must be constructed, a relatively large vein is needed in the immediate vicinity of the radial artery, and this is not always present. In the alternative, if a vein more than a centimeter away is mobilized and brought over to the artery, venous kinking can occur which results in decreased flow and early thrombosis. In addition, mobilization of the vein disrupts the tenuous vasovasorum, the miniscule arteries that provide blood supply to the vein wal itself, which can result in fibrosis of the vein wall and constriction of the vein lumen. This sets the conditions for early fistula failure.
Note also that each of the procedures described above must be done in the operating room. Most of the patients thus receive intravenous sedation and must be monitored postoperatively in a recovery room environment. Some remain hospitalized for a day or more as per the surgeon's preference. It is well known that individuals with renal failure exhibit impaired wound healing and a compromised immune function. These patients are therefore at increased risk for developing postoperative wound complications.
The conventional and limited usage of specialized catheters as exemplified by the medical practice of hemodialysis is thus well demonstrated and revealed. Clearly, despite the recognized desirability and advantage of creating an arteriovenous fistula for long-term or life-use patients needing dialysis, the use of catheters has remained limited and used primarily for the introduction and removal of fluids while the creation of arteriovenous fistulae remains the result of skilled surgical effort alone. Thus, although there is a long standing and well recognized need for an improved procedure and/or vehicle for generating arteriovenous fistulae, no meaningful alternative has been developed to date; and no catheter-based methodology or protocol has ever been envisioned as suitable for on-demand generation of an arteriovenous fistula in-vivo.